Coiled tubing is commonly used in a wide range of oilfield services and operations. It can be run into and out of a well bore at a high rate, relative to straight, jointed pipe, and, unlike wire line, it can be pushed into the well bore. It can be used to particular advantage in highly deviated wellbores found in extended reach drilling. Coiled tubing refers to a continuous string of steel pipe that is continuously milled and coiled onto a large take-up reel for transportation and handling. It can be used, for example, for drilling, but it is more often used after the well is drilled for logging, cleanouts, fracturing, cementing, fishing, completion and production related operations.
Coiled tubing is run in and out of well bores using machines called coiled tubing injectors. The name “coiled tubing injector” derives from the fact that, in preexisting well bores, the tubing may need to be forced or “injected” into the well through a sliding seal to overcome the pressure of fluid within the well, until the weight of the tubing in the well exceeds the force produced by the pressure acting against the cross-sectional area of the pipe. However, once the weight of the tubing overcomes the pressure, the injector must hold the tubing.
There are a number of different types and configurations of coiled tubing injectors capable of handling coiled tubing used in oilfield operations. Most coiled tubing injectors have a head comprised of two continuous chains, each mounted on sets of spaced-apart sprockets, so that there is an extended length of chain between the sprockets. One or more motors drive or turn the chains. Typically the motors are hydraulic, though other types of motors can be used. Each motor is connected to one or more of the sprockets. The chains are arranged so that the coiled tubing entering the injector is held between the chains by grippers mounted to each of the chains. The grippers are pressed against the outer diameter of the tubing thereby generating a frictional force parallel to the axis of the tubing. The frictional force is directly related to the normal force applied by the grippers.
The coiled tubing injector, a reel of coiled tubing, a control cabin, and equipment for operating the injector are typically transported to a well site as a single unit—a “coiled tubing unit” (CTU). Typically, the coiled tubing unit is transported to a well site to perform well intervention work of various types, usually as components mounted on a truck, trailer or several trailers, depending on the unit configuration. The riser is made one or more sections of straight pipe that extends from the blow out preventers attached to the wellhead. The riser accommodates elongated, rigid tools that are attached to the end of the coiled tubing prior to being lowered into the well bore. The connections between these components must comprise bolted flanges at pressures above 10,000 pounds per square inch (PSI) (approximately 89 MPa). At lower pressures connections between the components can be made with quick unions. The coiled tubing injector is connected to the riser with a stripper, through which the coiled tubing is pushed or pulled. Because there is no derrick or platform, a mobile crane is typically driven to the site to assist with holding the coiled tubing injector high enough so that it can be attached to the top of the BOP assembly. The crane is also used to carry the weight of the coiled tubing injector and BOP assembly, as well as to counter the bending moment on the wellhead caused by the tension placed on the coiled tubing by the reel. However, a temporary structure could also be erected above the wellhead for these purposes. A mast as disclosed in U.S. Pat. No. 7,077,209 could also be used.